Frequency error detectors are used for automatic frequency control (AFC) in control loops that are in radio transmission receivers, for example, for coherently modulated signals (e.g., quad amplitude modulation, QAM) to adjust the carrier frequency of the receiver to the carrier frequency of the transmitter, i.e., the carrier frequency of the received signal. In addition, these receivers for coherently modulated signals often have another closed loop for carrier phase synchronization which is capable of synchronization in the case of relatively small frequency deviations. The frequency error allowed in synchronization of the carrier phase is called the lock-in range. Since the transmission carrier frequency is not known at the start of a transmission or after a system failure, the receiver must be capable of reliably estimating this frequency. The accuracy of this estimation must be within the lock-in range of the carrier phase synchronization. The size of this lock-in range depends on several system parameters of the broadcast system. Narrow-band broadcast systems with a high number of modulation levels have a smaller lock-in range, but broad-band systems with fewer modulation levels have a larger lock-in range.
A conventional frequency error detector includes a suitable device that synchronizes the carrier frequency of the receiver at the transmission carrier frequency. This conventional device measures the differential power of the received signal spectrum symmetrically with the center of the band. An error signal generated in this way will disappear at the correct carrier frequency in the receiver. A quadricorrelator as descrived in German Patent No. 37 07 762 is one such frequency error detector. This quadricorrelator reacts very sensitively to signal distortions; i.e., it supplies false information about the frequency deviation when the received signal spectrum is skewed because of channel distortion. Straight-line broadcast systems usually have great channel distortion. Therefore, the background art frequency error detector would not be suitable for a receiver in a transmission system where channel distortion is expected.
The object of the present invention is therefore to provide a frequency error detector that will supply the most accurate possible information about the deviation in the carrier frequency in the receiver in comparison with the transmitted carrier frequency regardless of channel distortions.